1. Field of the Invention
This invention relates to a method for the production of an absorbent resin excelling in durability. More particularly, this invention relates to a method for the production of an absorbent resin excelling in absorption capacity, absorption velocity, and durability in the wetted state, suffering only sparingly from ropiness of wetted gel, and excelling in liquid permeability.
Since this absorbent resin not merely enjoys inexpensiveness and simplicity of manufacture and great safety but also exhibits an outstanding performance in water absorption, it is highly useful as sanitary materials for sanitary goods, disposable diapers, etc., as water-retaining agents for agricultural and horticultural devices and afforestation devices, etc., and further as materials for a wide range of absorbent articles.
2. Description of the Prior Art
In recent years, absorbent resins capable of absorbing water in amounts tens to hundreds times as large as their own weights have been developed and have been finding extensive utility in the field of sanitary materials such as disposable diapers and sanitary napkins, in the field of agricultural and horticultural activities, in the field of civil engineering, etc.
The absorbent resins of this class heretofore known in the art include partially neutralized cross-linked polyacrylic acid (Japanese Patent Laid-Open SHO 55(1980)-84,304), a hydrolyzed starch-acrylonitrile graft polymer (Japanese Patent Publication SHO 49(1974)-43,395), a neutralized starch-acrylic acid graft polymer (Japanese Patent Laid-Open SHO 51(1976)-125,468), a saponified vinyl acetate-acrylic ester copolymer (Japanese Patent Laid-Open SHO 52(1977)-14,689), a hydrolyzed acrylonitrile copolymer or acrylamide copolymer (Japanese Patent Publication SHO 53(1978)-15,959), and cross-linked products of such hydrolyzates, for example.
The characteristics which these absorbent resins are desired to possess include a high absorption capacity and a highly satisfactory absorption velocity to be exhibited on contact with an aqueous liquid and a highly satisfactory aspirating force for aspirating an aqueous liquid from a substrate containing the liquid.
Depending on purposes for which these absorbent resins are used, the absorbent resins are required to possess an ability to produce wet gels having durability and stability enough to withstand the effects of aging in addition to the characteristics mentioned above. When a conventional absorbent resin is used as a sanitary material in disposable diapers, for example, the wet gel which the absorbent resin produces on absorption of urine is deteriorated and decomposed by aging. The absorbent resin is also liable to undergo deterioration and decomposition after protracted use in agricultural and horticultural devices.
The methods heretofore known as capable of preventing the wet gel of absorbent resin include a method which causes the absorbent resin to incorporate therein an oxygen-containing reducing inorganic salt or a radical chain inhibitor (Japanese Patent Laid-Open SHO 63(1988)-118,375 and Japanese Patent Laid-Open SHO 63(1988)-152,667), a method which causes the absorbent resin to incorporate therein an oxidizing agent (Japanese Patent Laid-Open SHO 63(1988)-153,060), and a method which causes the absorbent resin to incorporate therein a sulfur-containing reducing agent (Japanese Patent Laid-Open SHO 63(1988)-272,349), for example. These methods invariably rely on the incorporation of a varying additive in the absorbent resin for the prevention of the absorbent resin from deterioration. By reason of the inevitable incorporation of additives, these methods are not necessarily desirable in terms of safety in the light of the fact that the absorbent resin is used as sanitary materials.
There is a method which imparts heightened gel strength and enhanced durability to the absorbent resin by increasing the amount of a cross-linking agent to be used and consequently heightening the cross-link density of the absorbent resin. This method, however, is such that the absorbent resin in its finished state suffers from an extremely low absorption capacity, because of a high cross-link density. Such is the true state of art. An absorbent resin which excels in safety, exhibits a high absorption capacity, and possesses outstanding durability remains yet to be developed.
In addition to the inferior durability mentioned above, the ropiness of the wet gel and the degradation of liquid permeability which occur when the absorbent resin is incorporated in a diaper have often, if not always, raised a problem. The absorbent resin sometimes contains a portion of the resin to be soluble in water (hereinafter referred to as a xe2x80x9cwater-soluble portionxe2x80x9d). This water-soluble portion imparts ropiness to the wet gel which the absorbent resin produces on absorption of aqueous liquid. When the absorbent resin is incorporated in such an absorbent article as a disposable diaper, this ropiness impairs the liquid permeability to the resin. The diaper, therefore, has a disadvantage that freshly discharged urine leaks from the diaper, for example. Since the content of the water-soluble portion in the absorbent resin is directly proportional to the absorption capacity of the resin, however, such a decrease in the content of the water-soluble portion inevitably entails an increase in the cross-link density of the absorbent resin. Thus, the absorbent resin having a small content of the water-soluble portion exhibits a proportionately low absorption capacity.
The absorbent resin has been known in the art to be such that the durability thereof is improved in proportion as the amount of the cross-linking agent incorporated therein is increased. Further, a technique for enhancing absorption capacity has been disclosed by using a chain transfer agent when the absorption resin is produced (U.S. Pat. No. 4,698,404). It, however, has the problem that the absorption capacity thereof is degraded in proportion as the content of the cross-linking agent is certainly increased. When a chain transfer agent is used in the production of the absorbent resin, it improves the absorption capacity of the absorbent resin with respect to water or physiological saline solution and brings about virtually no discernible improvement in the absorption capacity with respect to human urine.
An object of this invention, therefore, is to provide a method for the production of an absorbent resin excelling in durability.
Another object of this invention is to provide a method for the production of an absorbent resin which exhibits a high absorption capacity with respect to urine as well as to physiological saline solution, displays outstanding durability while in use in a disposable diaper, suffers only sparingly from back flow of absorbed urine in the diaper, gives rise to a wet gel of scanty ropiness, and excels in perviousness to liquid.
The objects described above are accomplished by a method for the production of (D) an absorbent resin excelling in durability, which method comprises polymerizing an aqueous solution containing 30% by weight to saturated concentration of (A) an aqueous water-soluble ethylenically unsaturated monomer solution containing (B) 0.005 to 5 mol % of a cross-linking agent and (C) 0.001 to 1 mol % of a water-soluble chain transfer agent, both based on the amount of the monomer (A).
The objects are further accomplished by a method for the production of (F) an absorbent resin excelling in durability, which method comprises polymerizing an aqueous 30% by weight to saturated concentration of (A) an aqueous water-soluble ethylenically unsaturated monomer solution containing (B) 0.005 to 5 mol % of a cross-linking agent and (C) 0.001 to 1 mol % of a water-soluble chain transfer agent, both based on the amount of the monomer (A) thereby preparing (D) an absorbent resin and cross-linking the surface region of the absorbent resin with (E) a hydrophilic cross-linking agent capable of reacting with the functional group of the absorbent resin.
The absorbent resin which is obtained by this invention, unlike that obtained by the conventional method, (a) excels in durability of wet gel, (b) exhibits a high absorption capacity particularly with respect to human urine and contains a soluble portion of low molecular weight, (c) enjoys a notable improvement in freedom ropiness of wet gel and liquid permeability, and (d) proves itself to be safe. In accordance with the method of this invention, the absorbent resin of such excellent quality can be manufactured simply by polymerizing (A) a water-soluble ethylenically unsaturated monomer in the presence of (B) a specific amount of a cross-linking agent and (C) a specific amount of a water-soluble chain transfer agent. The absorbent resin (F) can be obtained with notable improvement in absorption velocity and durability by cross-linking the surface region of the absorbent resin (D) with (E) a specific hydrophilic cross-linking agent. Since the absorbent resin can be produced inexpensively and possesses heretofore unattainable excellent absorption characteristics, it is useful extensively in the fields of sanitary materials, food processing materials, materials for civil engineering, and materials for agriculture, for example.
The absorbent resin of this invention and the method for the production thereof enjoys the following characteristic features.
(1) Heretofore, it has been customary to decrease the content of the cross-linking agent in the polymerization mixture for the purpose of enabling the produced absorbent resin to acquire a high absorption capacity. The wet gel of this absorbent resin, therefore, is deficient in durability. In accordance with the method of this method, the produced absorbent resin possesses a high absorption capacity particularly with respect to human urine even when the cross-linking agent is used in a large amount. The wet gel, therefore, exhibits outstanding durability.
(2) Even when the monomer to be used is polymerized in a high concentration near its saturated concentration, since such unnecessary reactions as self-crosslinking are suppressed and the absorbent resin is obtained with high performance, the monomer can be polymerized in a high concentration and the absorbent resin can be manufactured with high productivity.
(3) The absorbent resin of the present invention has a low molecular weight in the water soluble portion which automatically occurs in the absorbent resin and, therefore, exhibits low viscosity, the soluble portion is incapable of producing such adverse effects as impartation of ropiness to the wet gel and degradation of perviousness of the resin to liquid.
(4) The cross-linking of the surface region of the absorbent resin contemplated by the present invention is so effective in improving absorption characteristics to the extent never attained in the surface cross-linking of the conventional absorbent resin that the absorbent resin undergone the cross-linking treatment exhibit further improved durability and absorption velocity.
(5) In accordance with the conventional method, when the amount of the polymerization initiator is increased for the purpose of decreasing the unaltered monomer content and shortening the induction period and the polymerization time, the absorbent resin cannot be produced with a high absorption capacity because the polymerization entails such unnecessary reactions as self-crosslinking. In accordance with the method of this invention, the absorbent resin can be obtained with a high absorption capacity and a low unaltered monomer content even when the amount of the polymerization initiator is increased.